Process for the production of high alpha cellulose fiber



G. A. RICHTER Ja'n. 7, 1930.

PROCESS FOR THE PRODUCTION OF HIGH ALPHA CELLULOSE FIBER Filed April 28, 1925 wqk harm n AMS Patented Jan. i7, 1930 UNITED STATES PATNT orricia:

GEORGE A, RICHTER, F BERLIN, NEW Il'IAllCEISBIIRE, ASSIGNOR T0 BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE PROCESS FOR THE PRODUCTION OF HIGH ALPHA CELLULOSE FIBER Application led VApril 28, 1925. Serial No. 26,361.

This invention relates to the utilization of the heat and the recovery of the sodium constituent from the spent liquor resulting from the treatment of sulphite pulp in the manufacture of a product containing a high percentage of alpha-cellulose.

High alpha-cellulose fiber is produced by the alkaline digestion of a predigested pulp such as unbleached sulphite pu1p,-a. solution lo of caustic soda, or of soda ash, or a solution of mixed Vcaustic soda and soda ashbeing especially adapted for the purpose of digestion. This digestion may take place in digesters at any suitable temperature and pressure, although I have found it more economical and expedient in many cases to effect the digestion in open tanks at from about 180 F. to 212 F; under atmospheric pressure. The alkaline solution, reacts 'with 2u and dissolves the lignin and the less resistant beta and gamma celluloses contained in the sulphite pulp, and7 after a suliicient period of digestion, yields a product which contains about 94% alpha-cellulose and, after bleaching, is very white. To render the process more economical, the sodium constituent and the available heat content lof the spent digesting liquor should be recovered.

The object of this invention is to provide an economical process for the utiliz-ationV of the availablel heat and the recovery of the sodium content from the spent liquor resulting from such an alkaline digestion of sulphite pulp in the preparation of high alphacellulose liber. This object is attained by reemploying substantially all the spent liquor in the digestion offresh sulphite pulp in a manner which will best be understood from the'following description when considered in connection with the accompanying drawing, which is a diagrammatic illustration of the cycle of operations in the process.

As tliealkaline digestion of sulphite pulp is at present carried out, the recovery of heat and alkaline constituent from the spent liquor can not be economically performed. An analysis of the present method will explain the foregoing statement.

The present method is about as follows To the stock suspension resulting from Washing and' screening the sulphite pulp after it hasn been digested in a suitable sulphite liquor,'

as, for example, calcium bisulphite, is added the amount of caustic soda solutionnecessary to remove the lignin and the betaand gamma celluloses therefrom. The stock is then digested in open tanks atabout 180 to 212 F. with agitation for a prescribed period of time, thereby producing an alphacellulose fiber and yielding a spent alkaline liquor containing'about 3% dissolved solids.v This dissolved material comprises residual caustic soda, sodium carbonate, and various organic compounds which have been removed by reaction with the caustic soda or sodium carbonate., The spent liquor is at a .temperature of about 180 to 212 F., b ut is so dilute that it Wouldnot be economical to recover lthe sodium constituent directly therefrom.

'constituent therefrom.

' In order to produce spent liquor of such a concentration, it is necessary to re-employ the initial liquor in successive digestions of vthe sulphite pulp.- This is not possible as the process is practiced at present, since a stock of 10% to 12% concentration is deliv'ered to the digesting tanks. To this 10% to 12% stock only a smallportion of the hot spent dilute liquor could be added, since otherwise the stock concentration would be reduced to such a point that, With the addition of the usual quantity of alkaline digesting agent, digestion of the pulp to produce a product of the desired alpha cellulose content Would be rendered ineffective; but if a stock of a higher consistency were delivered to the digesting tanks, more hot spent liquor could be used in making up the digesting liquor, and the higher the concentration of stock fed, the more the amount of hot spent li uor that could be used.

q:Provision is therefore made according to the present invention, for increasing the consistency of the unbleached sul hte stock.

This may be accomplished, as or examp1e,

to a consistency of 30% to 50% by press rolls,

is continuously delivered to a digesting tank A communicating by a slow-moving worm with a second digesting tank B. The stock is diluted in the tank A to a consistency of about 10% with the required amount of caustic soda or soda ash solution of the proper strength, the worm operating to feed the stock to the tank B. Digestion of the stock is effected with agitation at about 180 F. to 212 F., as by blowing live steam into the tanks. In the reaction taking place during digestion, the less resistant cellulose and the lignin components of the pulp react with and are dissolved by the caustic soda solution, so that after a prescribed period of digestion there results a product consisting essentially of resistant or alpha-cellulose. The agitation during digestion promotes intimate contact between the pul and the solution, effecting a more thoroug and uniform reaction so that a uniform reaction product results. After alkaline digestion, the high alpha-cellulose stock While still at elevated temperature is passed through a counter-current washer and then to the bleachery. The Wash Water acquires practically the same temperature as the pulp and the hot spent digesting liquor and, inasmuch as it contains the components dissolved from. the pulp and the chemicals, it constitutes the spent liquor of digestion. All the hot wash water or spent liquor from the washing apparatus is passed into a liquor storage tank and comprises in volume ap roximately the volume of Water used in ma ing up the 10% stock suspension, plus the total Wash water minus that portion absorbed by the digested pul plus the water content of the initial su phite pulp, plus the small additional amount due` to the condensation of steam used in heating the alkaline liquor and the sulphite pulp in the digesting tanks. A

When the process is initially started, the amount of Water used in thecountencurrent washer is such that the volume of hot spent liquor delivered from the washer to the storage tank is substantially equal to that used in making up the original 10% stock suspen! sion and the resulting hot wash water or spent liquor is passed back to the digesting tanks. A sufficient quantity of 50% sulphite pulp necessary to make up a 10% suspension is fed into the digesting tanks along with the hot vWash water, and an amount of caustic soda or soda ash necessary to react upon and dissolve the less resistant celluloses and the lignin is added in the form of a concentrated solution to the suspension.v The stock is digested, and the cycle of operations as hereinbefore described is repeated, the resulting hot spent liquor containing a higher percent of dissolved solids. The process is preferably a continuous one, a stream of 10% stock being continuously charged into tank A and continuously digested therein, and the stock being fed from tank A into tank B., and further digested therein the digested stock being continuously withdrawn from tank B at a rate equal to that at which it is being charged into tank A and the time of digestion of the stock hence being equal to the time re uired for its complete passage through oth digesting tanks. Or the process may be a batch one if desired. In any case it is repeated or continued until a hot syrupy spent liquor of about 10 to 15 Baume` results, as such a liquor may profitably be evaporated and its combustible content burned to permit recovery of the valuable inorganic or sodium content. When this stage is reached, the flow of fresh water to the washer is increased. As stated previously, up to this stage the volume of spent liquor delivered from the washer has been only sufficient to dilute the incoming stock to a 10% consistency (or suflicient to ensure such consistency when the volume of solution containing fresh chemical is taken into account), but there is now an excess of spent liquor and this excess is continuously withdrawn from the spent liquorLtank and passed to the evaporators. The increase in quantity of fresh water is so adjusted as to maintain an equilibrium within the system at this particular concentration: i. e., the excess spent liquor passed to the evaporators contains substantially the amount of spent digesting chemical formed in a preceding digestion. The liquor Withdrawn from the spent liquor tank is evaporated to the desired degree and is burned in a furnace under oxidizing conditions, the eiiiuent smelt from the furnace being run into a relatively small amount of Water contained in a dissolving tank to produce a concentrated solution, which is maintained at the boiling point by the heat of the smelted sodium compounds. The smelt consists essentially of soda ash (sodium carbonate), the

lboiling solution thereof resulting in the dissolving tank being delivered into the digesting tank A. A new concentrated solution -containing caustic soda or soda ash equivalent to that lost in the cycle, mal);

be added to thedigesting solution so that w en the prop` er amount of sulphite pulp is added a 10% suspension results.

Although the stock consistencies indicated lon the drawing and hereinbefore stated are to be preferred for optimum operating-conditions, it is evident that, in the event diculty is experienced in handling a 10% stock in thedigesting tanks, stocks of different' consistencies may be employed.- For example,

a 16% sulphite stock may be delivered to the- 'digesting tanks and then diluted by liquor to a 3% consistency. Insuch case, however, a longertime would be required to reach conditions of equilibrium, and much largerapparatus would benecessary to Vmaintain the capacity of a plant.

as presented by the dash lines. If desired,the

the recovered sodium carbonate may-'be converted into caustic soda by'causticization with lime. The precipitated i calcium carbonate may then be removed (by filtrationy as usual, and the caustic soda solution passedl into the digesting tanks. If this mode of operation were carried out and caustic soda were-addedto the tanks for `making up sodiumiosses,

digestion would take place in a solution containing only caustic soda.

The main cycle, shown in solid lines, hojvever, is more economical and is simpler to practice. The sodium losses inthe cycle may be made up by the addition of soda ash to the digesting tanks, in Which case the digesting liquor would contain substantially only `sodium carbonate. Or such losses maybe made up by the addition of caustic soda, in which case the digesting liquor would consist of a mixture lcontaining sodium carbonate anda small amount of caustic soda. v

A process practiced as hereinbefore outlined is very economical. The sensible heat of the cooking liquor and of the moltenrecovered sodium carbonate is conserved, and 'although' a 10% stock suspension is cooked, theoretically substantially no more heat is used in cooking than would be required if a 50% stock suspension, or even stock, were cooked. It is impossible .to cook a 100% stock, and it is very diiiicult to cook a 50% stock with ysuch alkali as is used, owing to thetremendous .power which would be needed in moving suchv stock, as well as the impossibility of obtaining uniform and intimate mixture of caustic soda solution with such thick stock.

With the process described and assuming a perfect counter-current washer it is theoretically possible to recover or conserve all of the chemicals and the available heat of the spent liquorleaving the reaction tank with the digested'pulp. For example, assuming a feed of a stock of 50% consistency to the reaction tank A, a 10% suspension or concen-v The drawing shows an alternative methodJ tration in the'reaction tank, andI aV stock'con- Y .such conditions, theoretically about one ton of water is heated per ton of bone-dry stock ed. In actua-l practice, however, these theoreticalconditions. are not maintained, and consequently the proportionate volume of wash water, and the proportionate volumes of vwater delivered to thereaction tank with the stock and with the make-up raw materials `may vary from those stated within reasonable limits. It willfbe understood vthat with an increasing concentration of'pulp delivered to the reaction tankthere is a decreasing dilution'of the recovered spent liquor delivered to such tank from'the washer, and a greater conservationand utilization of the heat of such spent liquor in the process ofdigestion. By. employing a stock concentration of 50%, an economlcal conservation and utilization of the heat of such spent liquor results. 4

By my process a large portion of the lunconsumed caustic soda, whichmay possibly amount tol 20% to 30% of the original quantity added, is returned tothedigesting tanks.

When equilibrium in the process has been reached,.substantially all the sodium compounds are recovered in the form of sodium carbonate or sodium hydroxide. Furthermore, the heat of combustion of the'dissolved organic matter, in the form lof the sensible 'heat of the sodium carbonate smelt, which is delivered tothe dissolvingtank and sent back to the digesting tank in the form of a concentrated and boiling sodiumcarbonate Solution, is recovered.

I do not herein claim the subject matter of my applications'Serial Nos. 26,362, 28,339, and 29,713, respectively, filed April 28, 1925, May 6, 1925, and May 12, 1925, which disclose a process such as herein described, practised in inter-relation with fiber-liberating processes.

I claim:

1. A process which comprises indefinitely repeating the steps of digesting predigested pulp in'an alkaline liquor and of re-employing substantially all the spent liquor in anl other digestion of pulp afterthe addition of suii'icient digesting-agent thereto to make up losses therein for effective digestion.

2. A process of producing high alphacellulose pulp from sulphite pulp, which comprises indefinitely repeating the steps of digesting such pulp under heat in a hot alkaline liquor, of separating the spent liquor from the digested pulp in hot condition, and of re-employing substantially all the hot spent liquor in another digestion of pulp after the addition of suiicient digesting agent thereto to make up losses therein for effectivedigestion.

8. A process which comprises digesting predigcsted pulp in an alkaline liquor, reemploying substantially all the spent liquor in another digestion after the addition of s uf- Iicient digesting agent thereto, continuing suoli re-eniployineiit until a predetermined concentration of dissolved material results therein, and thereafter recovering the digesting agent from a relatively small p ortion o spent liquor containing substantially the amount of spent digesting constituent which was formed in the preceding digestion, and re-employing the relatively arge portion in another digestion.

4. A process which comprises digesting predigested pulp in an alkaline liquor, reemploying substantially all the spent liquor in another digestion after the addition of sufficient digesting agent thereto, continuing such re-employnieiit until a 1()o to 15O Baume spent liquor results, and thereafter recovering digesting agent from one portion of the spent liquor and re-employing the other portion in another digestion.

5. A process which comprises digesting predigested pulp in an alkaline liquor, reeniploying substantially all the spent liquor in a new digestion after the addition oi: a sufficient digesting agent thereto, continuing such re-employment until a predetermined concentration of dissolved material results therein, and thereafter re-employing one portion of spent digesting liquor inanother digestionh recovering digesting agent from another portion containing substantially the amount of spent digesting agent Which Was formed in the preceding digestion, and adding said recovered agent to the spent liquor to be re-employed.

6. A cyclic process which comprises digesting predigested pulp in an alkaline liquor, re-employing substantially all the spent liquorin another digestion after the addition 'of suilicient digesting agent thereto, continuing such re-employinent until a predetermined concentration of dissolved material results therein, and thereafter rie-employing one portion of spent digesting liquor in another digestion, recovering digesting agent from another portion containing substantially the amount of spent digesting agent which was formed in the preceding digestion, adding said recovered agent to the spent liquor to be ie-employed, and making up losses in the cycle by the addition of digesting agent to the re-employed liquor.

7 The process of producing a high alphacellulose fiber, whiclL comprises indefinitely repeating the steps of digesting pi'edigested pulp in an alkaline liquor containing sodium compounds, of separating the spent-liquor from the 'digested pulp in hot condition, and re-employing substantially all the spent liquor of digestion in a'nother digestion of pulp after the addition of suiiicient sodium constituent thereto to make up losses therein.

8. The process of producing a high alphacellulose fiber, which comprises digesting piedigested pulp in an alkaline solution containing sodium compounds, re-employing substantially all the spent liquor in another digestion after the addition of sufiicient sodium digesting agent thereto, continuing such re-employmeiit of such spent liquor until a predetermined sodium concentration results therein, and thereafter recovering digestion agent from a relatively small portion of the spent liquor, and re-employing the relatively large portion in another digestion.

9. The process of producing a high alphacellulose fiber, which comprises digesting pi'edigested pulp in an alkaline solution containing sodium compounds, re-einploying substantiall all the spent liquor in another digestion o sulphite ulp after the addition of sufiicient sodium igesting agent thereto, continuing such re-eniployment until a predetermined sodium concentration results therein, and thereafter recovering spent sodium digesting agent from one portion of spent digesting li uor containing substantially the amount o spent sodium constituent which was formed in the preceding digestion, and re-employing the other portion in a subsequent digestion.

10. A process which comprises digesting sulphite pulp in a hot alkaline digesting liquor, recovering digesting agent from a relatively small portion of hot spent digesting liquor and re-eniploying the other portion of the spent liquor together with suiicient alkali to make up losses in another digestion of sulpliite pulp.

11. A process of producing a high alphacellulose fiber, which comprises digesting predigested pulp in an alkaline liquor containing sodium compounds, recovering sodium constituent from one relatively small portion of the spent liquor, and re-employing the other portion of the spent liquor in another digestion of predigested pulp.

12. The process of producing a high alphacellulose fiber, Which comprises heating and digesting sulphite pulp in an alkaline liquor containing sodium compounds, recovering the sodium constituent from a smaller portion of the spent liquor containing substantially the amount of spent sodium digesting agent which was formed in the preceding digestion and re-employing the larger portion of the hot spent liquor in another digestion of sulphite pulp.

13. A cyclic process of producing a high alpha-cellulose ber, which comprises heatingand digesting sulphite pulp in an alkaline liquor containing sodium compounds, reemploying a larger portion of hot spent digesting liquor in another digestion, recover ing sodium digesting agent from the other portion of the spent liquor, adding said recovered agent to the re-employed liquor, and making up losses in the cycle by the addition of digesting agent to the re-employed liquor.

14. A continuous cyclic process which comprises continuously feeding) and digesting predigested pulp in an alkaline liquor, Washing the resulting pulp and the entrained products of the reaction, circulating a part of the Wash water back for further digestion of like pulp, continuously evaporating the other portion of said Wash Water, burning the combustible content thereof, and smelting the inorganic alkali content thereof, recovering'lthe molten smelt in liquor, and mixing the resulting liquor with the recirculated Wash water. 15. A continuous cyclic process of treating sulphite pulp for the production of high alphacellulose pulp, which comprises continuously feeding and digesting such first mentioned pulp under heat in a hot alkallne liquor, Washing the resulting pulp and the entrained products of the reaction, circulat ing a part of the hot Wash Water back for further digestion of sulphite pulp, continuouslyr evaporating the other portion of said Wash Water, burning the combustible content thereof, and smelting the inorganic alkali content thereof, recoverlng the molten smelt in liquor, and mixing the resulting hot liquor with the recirculated wash Water, thereby conserving heat resulting from such digestmn and such smelting. In testimony whereof I have affixed my signature.

' GEORGE A. RICHTER. 

